The brace connects the two shock absorber mounts. In a fast corner, the loaded side of the suspension pushes outward and the geometry deteriorates at precisely the moment you need it most: the brace distributes the load between both sides.

The front end responds more precisely when turning in, and the car maintains the same response from one corner to the next. The effect is clear on older chassis, convertibles and high-mileage cars; it is more subtle on an already rigid modern body.